16.04 – Purge PPA after wrong distro in sources.list

I accidently added Debian PPA to my sources.list, and upgraded, and now have a broken Ubuntu 16.04 desktop.

I looked at my sources.list entry and everything has xenial sources except

deb http://deb.debian.org/debian buster main contrib non-free.

My other usual entries are of the the form deb archive.ubuntu.com/ubuntu xenial main.

How do I purge the Debian packages and recover Ubuntu desktop?

I tried sudo ppa-purge -s http://deb.debian.org/debian debian/buster, but it says no package list for PPA: debian PPA.